1. Field of the Invention
The present invention relates to an imaging apparatus offering a dynamic range that is expandable by weighting a plurality of image signals, which are produced during different exposure times or the like, with a plurality of coefficients whose sum is 1, and adding them up.
2. Description of the Related Art
In general, a range of luminance levels within which imaging is validated by an imaging apparatus such as a TV camera is determined unconditionally with the photoelectric conversion characteristic of an imaging means, for example, a solid-stage imaging device.
Specifically, a lower limit of output levels of a solid-state imaging device is determined by a noise level. An upper limit thereof is determined by a saturation value. An operation range within which the solid-state imaging device is usable is thus defined. The slope of a characteristic curve expressing the output levels of the solid-state imaging device is fixed to a certain value. Eventually, the range of luminance levels within which imaging is validated by the solid-state imaging device is determined unconditionally.
For example, Japanese Unexamined Patent Publication No. 57-39673 has disclosed an imaging apparatus offering a dynamic range, which is expandable by synthesizing image signals produced at two different luminance levels, for an image signal produced by a solid-state imaging device.
However, according to the prior art, the dynamic range offered by the imaging apparatus is expanded merely by adding up two image signals or subtracting one image signal from another. A signal-to-noise ratio of a component of a resultant signal indicating a low luminance level deteriorates. A synthetic picture signal produced by synthesizing two image signals relative to luminance levels is plotted as joined straight lines or a graph of broken lines. This means that the synthetic image signal varies according to the graph of broken lines. Since an output level of a solid-state imaging device varies at a luminance level at which the straight lines are joined, a constructed color image does not depict a smooth color change but gives a sense of incongruity.
Moreover, according to Japanese Unexamined Patent Publication No. 6-141229, two or more image signals produced during different charge accumulation times are weighted based on the signal levels, and compressed by a compressing means. Thus, a picture signal whose components range from a component representing a dark image to a component representing a bright image will not have a saturation value that can be constructed with few noises.
In the prior art, as shown in FIGS. 4 and 9 in the patent publication, a plurality of image signals produced during different charge accumulation times are weighted by varying a weight under a boundary condition, under which one of the image signals has a saturation value, or thereabout. Resultant image signals are then synthesized by adding them up. The image signals are then compressed by a compressing means according to a frequency band. Eventually, a synthetic picture signal exhibiting a desired input/output characteristic is constructed.
However, according to the prior art, for example, two image signals are used to construct a synthetic picture signal. At this time, the synthetic picture signal is constructed using only a first image signal under conditions lower than the boundary condition under which the first image signal produced during a longer charge accumulation time has a saturation value.
The synthetic picture signal is compressed on a subsequent stage. Since one synthetic picture signal resulting from addition is compressed, the foregoing characteristic cannot be changed. For example, when an image signal represents a motion, an image reconstructed may suffer from inconsistency between contours represented by a signal component indicating a low luminance level and a signal component indicating a high luminance level. Besides, the image has a portion in which colors change unnaturally.
An object of the present invention is to provide an imaging apparatus capable of offering an expandable dynamic range without any deterioration of a signal-to-noise ratio of a signal component indicating a low luminance level, and of constructing a smooth image giving no sense of incongruity.
Another object of the present invention is to provide an endoscopic imaging apparatus capable of offering an expandable dynamic range without any deterioration of a signal-to-noise ratio of a signal component indicating a low luminance level, and of constructing an image suitable for diagnosis.
An imaging apparatus in accordance with the present invention includes, an imaging means for successively outputting a first image signal, which is produced by imaging an object during a first exposure time, and a second image signal produced by imaging the object during a time shorter than the first exposure time. The invention includes a picture signal producing means composed of first and second weighting circuits for applying a first weight, which decreases monotonously with an increase in light level, and a second weight, which increases monotonously therewith, to the first and second image signals within the range of light levels up to light levels, at which the first and second image signals have a saturation value, under the condition that the sum of the first and second weights is 1. Also included is an adding circuit for adding up first and second picture signals produced by applying the first and second weights to the first and second image signals by means of the first and second weighting circuits. In addition, a signal processing means is included for processing a picture signal output from the adding circuit to produce a video signal based on which an image can be displayed on a display means.
Owing to the above components, the ratio of the first image signal to the second image signal can be modified, and the image signals are synthesized with each other. Consequently, the dynamic range for a picture signal is expanded, and deterioration of a signal-to-noise ratio of a signal component indicating a low luminance level is prevented. This results in a natural smooth image not giving a sense of incongruity.
Moreover, when the present invention is adapted to an endoscopic imaging apparatus, an image helpful for diagnosis can be provided.